The expression of the cell cycle regulating gene, cdc2, during the in vitro activation of murine resting T cells by various ligands as well as in continuous growing, IL-2 addicted cytotoxic T cells was investigated. The expression of cdc2 in T cells is linked to the T cell receptor and is dependent on the presence of IL-2 during the activation process. T cells activated in the absence of IL-2 fail to express the gene and accumulate in G1. Withdrawal of IL-2 from continuously growing IL-2 addicted cells which constituitively express cdc2 results in an early reduction in specific cdc2 mRNA and loss of detectable kinase activity. These IL-2 starved cells accumulate in G1. Readdition of rIL-2 stimulates an increase in specific cdc2 mRNA and the cells return to a normal IL-2 dependent proliferation cycle. Withdrawal of IL-2 also results in the rapid dephosphorylation of tryrosine residues in at least 7 intracellular proteins. The addition of rIL-2 reestablishes the phosphorylation pattern in all proteins but most strikingly in a 97 kDa protein. In activated T cells, the only phosphoprotein that was significant phosphorylated on tyrosine residues after exposure to rIL-2 was also 97 kDa. Evidence suggest that the IL-2 mediated proliferation involves the tyrosine phosphorylation of this protein.